Saied Baghban, PhD

As pathways for mineralizing fluids and recording the evolution of the ore-forming systems, veins are critical features in magmatic-hydrothermal ore deposits. The Mount Pleasant deposit, with its successive intrusive phases and diverse mineralization styles, contains a complex veining system that documents the history of fluid evolution and metal saturation. These deposits are associated with three phases of evolved, Late Devonian A-type granites: fine-grained granite I (hosting W-Mo-Bi), porphyritic granite II (hosting Sn-Zn-Cu-In), and medium-grained granite III, remains unlinked to known mineralization. Vein types and their sequence for each mineralization phase are established by examining mineralogy, crosscutting relationships, and transitions from high- to low-temperature, relatively reduced to more reduced conditions, and low- to high-ƒS₂. Rare earth elements and yttrium (REY) in these vein–veinlets are identified using µXRF-EDS mapping, SEM-BSE and EDS imaging, and cathodoluminescence. During the first mineralization episode, ore deposition is mostly within breccia pipes, vein-veinlets, and stockwork zones. This stage is associated with greisen I (fluorite+topaz+quartz+sericite) alteration, whereas lower-grade ore intervals correspond to greisen II (topaz+quartz+chlorite+biotite). From early to late, the vein–veinlet sequence includes quartz stockwork, wolframite-arsenopyrite, wolframite-native bismuth-arsenopyrite-löllingite-quartz, wolframite-molybdenite-bismuthenite-löllingite-quartz, wolframite-molybdenite-quartz-fluorite, löllingite-fluorite, molybdenite-quartz-fluorite and quartz-fluorite. Spatially and temporally, tungsten and molybdenum veins display a distinct zonation: earlier stages are dominated by distal tungsten-rich veins, with later molybdenum-dominant veins developing deeper and more proximally, often beneath tungsten-bearing zones. The second mineralization event occurs within breccia pipes, lode-vein-veinlet systems, tin-bearing greisen zones, massive sulfide-arsenide replacement zones, and miarolitic cavities, all of which are partially superimposed on the first mineralization stage. The vein-veinlet sequence comprises quartz stockwork, wolframite-arsenopyrite, cassiterite-arsenopyrite-chlorite, cassiterite-arsenopyrite-sphalerite-chalcopyrite-fluorite, sphalerite-chalcopyrite-cassiterite-stannite, galena-sphalerite-stannite-fluorite, sphalerite-tetrahedrite-quartz-fluorite, quartz-fluorite, fluorite-kaolinite, and rhodochrosite veins. This mineralization episode also has a distinct temporal-spatial zonation: tin-rich veins tend to form earlier and closer to the causative granite (mostly in an endogranitic setting), followed by later zinc-copper-lead vein-veinlets in distal settings. Minerals enriched in REY occur in late-stage molybdenite–quartz–fluorite and quartz–fluorite veins as monazite, xenotime, and REY-rich whitish-green fluorite, primarily associated with the first mineralization stage. Based on µXRF-EDS mapping, fluorite displays oscillatory zoning, with REY content increasing from core to rim. Under cathodoluminescence, REY-rich bands appear yellowish-orange to white, whereas REY-poor zones range from greenish-blue to reddish-brown. In conclusion, the Mount Pleasant deposit exhibits a clear temporal-spatial vein-veinlet zoning—transitioning from early W- and Sn-rich to later Mo-, Cu-, Zn-dominant—with REY minerals primarily concentrated in late-stage molybdenite–quartz–fluorite and quartz–fluorite veins.